Dew, Ice and Snow vs. Your Heating and Ventilation Unit: How to Protect It

A 40 cm icicle breaking off an eave 3 m up hits the fan grille hard enough to bend the heat exchanger fins or crack a blade. Service technicians see this kind of damage every winter, and the cause is water and where it falls. Protecting a heating and ventilation unit from dew, ice and snow therefore starts with working out where the water comes from and where it drains to. Only then should you choose an enclosure.

We covered frost on the heat exchanger and the defrost cycle in a separate article. Here we focus on what happens around the unit: the roof above it, the gutter, the ground beneath it and the surfaces where moisture condenses.

Where the water that freezes around the unit comes from

An outdoor unit running in heating mode is exposed to four independent sources of water. Each one needs a different kind of protection.

| Source | When it occurs | Typical effect | Protection |

|---|---|---|---|

| Defrost condensate | at around 0 to +5°C and high humidity | ice mound under the unit, frozen drain pan | free drainage, optionally a drain heater |

| Water from the roof and gutter | daytime thaw, nighttime frost | icicles, ice falling onto the casing | canopy with adequate load capacity, relocating the unit |

| Precipitation and freezing rain | all winter | iced-up fan grille, snow in the heat exchanger | roof cover, raising the unit above the snow cover |

| Dew | autumn and spring, mornings | moisture on the enclosure and supporting structure | suitable material, drainage from horizontal surfaces |

The amount of condensate depends on the unit's capacity, air humidity and the number of defrost cycles. Manufacturers don't always state it in the documentation, but on damp, frosty days a single unit can discharge several litres of water per day, and larger units more than ten. That water needs somewhere to go before it freezes.

How to protect a heating and ventilation unit from dew, ice and snow

Effective protection consists of three elements that only work together:

• Mounting height. The bottom edge of the unit should sit above the expected snow depth, plus a margin for drifting snow. In most of Poland, a minimum of 30–50 cm above ground is assumed; more in foothill areas and open terrain. The specific minimum value is given in the installation manual for your model.
• Roof cover. A canopy above the unit keeps off precipitation, freezing rain and water running down the façade. However, it must leave the clearance above the top of the unit required by the manufacturer. For units that discharge air upwards, this clearance is usually much larger than for horizontal discharge.
• Drainage underneath. The ground or base beneath the unit must be able to absorb water. A concrete slab with no slope and no drainage is the most common cause of an ice mound that reaches the condensate pan within a few weeks.

A side enclosure with louvres helps limit drifting snow, but only if the louvres slope downwards and outwards and their spacing doesn't restrict airflow. We described how to maintain performance when enclosing a unit in a separate article.

Ice and snow from the roof: the most overlooked hazard

Units are often mounted under the eaves because it's close to the wall penetration. But that spot collects all the water and snow from the roof slope. Three situations are particularly risky:

1. A unit under a gutter with no snow guards. Snow sliding off a metal roof can come down in a single mass weighing several dozen kilograms. No lightweight canopy will withstand that, and an unprotected unit can be buried up to half its height.
2. A leaking or clogged gutter. Water overflows the edge, forms icicles and drips directly onto the casing, where it freezes in layers.
3. Eaves above the discharge. Warm, humid air from some units (e.g. in cooling mode in summer or from ventilation) and cold air from a heat pump in heating mode change the conditions under the eaves, promoting icing of the soffit and gutter directly above the unit.

If the unit is already under the eaves, there are two solutions: install snow guards on the roof above it along with a canopy whose load capacity matches the snow load zone, or move the unit 1–1.5 m to the side, out of the eave line. The second option requires extending the refrigerant piping and should be carried out by an installer with F-gas certification.

Condensate drainage that doesn't freeze

Most units have a drain pan with a drain hole in the base. Some models designed for low-temperature operation come with a factory-fitted pan heater, others offer it as an option, and some don't provide for one at all. Check your unit's specification before looking for external solutions.

Regardless of the heater, drainage works properly when:

• water falls freely onto a permeable base, e.g. a 20–30 cm layer of washed gravel on geotextile,
• there is no horizontal slab under the unit where water can pool,
• the drain hose, if connected, runs downhill to a soakaway or storm drain and has no section above ground longer than about 10–15 cm; longer sections without a heating cable freeze at the first frost,
• the supporting structure doesn't block the holes in the drain pan.

If ice is already forming under the unit, don't chip it away with tools near the heat exchanger. It's better to pour warm, not hot, water over the mound with the unit switched off, and improve the base in spring.

Dew on the enclosure and structure: when it matters

In autumn and spring, when there's a large temperature difference between night and morning, dew settles on every metal surface around the unit. By itself it doesn't harm the unit, but it does matter for the structure around it:

• on galvanised steel with damaged coating, dew accelerates corrosion at cut edges and mounting holes,
• on untreated wood, it causes the louvres to swell and crack,
• on flat, horizontal canopies with no slope, dew combines with precipitation and freezes overnight into a thin layer of ice.

Anodised or powder-coated aluminium doesn't corrode from dew in any way that affects durability, and a canopy with a slope of a few degrees lets water run off before the temperature drops below freezing. Dew on the outdoor unit's heat exchanger itself is normal, and the unit handles it without issue.

What to check before the first frost

A short checklist to go through in October or November:

• is the gutter above the unit clear and leak-free,
• are there snow guards on the roof above the unit, if the roof is covered with metal sheeting or smooth-surfaced tiles,
• what is the distance from the bottom edge of the unit to the ground, and does it exceed the typical snow depth in your area,
• are the holes in the condensate pan clear, and is there no standing water under the unit,
• does the canopy or enclosure leave the clearances required by the manufacturer on every side,
• does the model have a pan heater, and is it connected.

Start by inspecting the installation site after the next rainfall: see where water runs off the roof and where puddles form under the unit. In practice, this shows which of the protections described above you need. Then compare it with your unit's installation manual, in particular the minimum clearances and the information about the pan heater. Only with this data in hand is it worth choosing a canopy or enclosure, because its dimensions depend directly on it.